The Guggenheim Museum in Bilbao, European flagship for the US cultural foundation, is reported to have paid well over the odds for art to fill Frank Gehry's swooping titanium masterpiece.

Documents relating to the acquisition of Guggenheim Bilbao's permanent collection between 1996 and 1998 reveal that the Guggenheim Foundation's New York boss and international supremo of the Bilbao museum, Thomas Krens, purchased works that caught his personal fancy while trawling the galleries and studios of artists he knew, Basque investigators say.

Mr Krens, with a budget of €96m (£76m) provided mostly by Basque regional authorities, paid more than the market value for the works, offering whatever price the artists demanded, a Basque regional MP who is examining the museum's records said yesterday.

As Richard Armstrong, former director of Pittsburgh's Carnegie art museum, prepares to become the Guggenheim Foundation's boss next month, allegations are flying around about the erratic way artworks were assembled for Guggenheim Bilbao by his predecessor.

One controversial acquisition questioned at the time by the Basque government's adviser to the museum between 1996 and 1998, Javier Gonzalez de Durana, was Large Blue Anthropometry, by Yves Klein. The museum paid $2.7m for the work, when it should not have paid more than $2m, Mr Durana wrote. He said Mr Krens built up Guggenheim Bilbao's collection on the basis of "personal criteria and overpayments".

Works by Cy Twombly and Brice Marden were earmarked as highlights for one of the museum's principal second-floor rooms. But those purchases fell through and, at the last moment, Mr Krens requested instead a work from Sol Lewitt, who sold him Wall Drawing 831.

"That commission was the result of an improvisation, since this artist was not among those considered important for the collection," Mr Durana wrote. He believed the Lewitt work, hailed by Mr Krens as a "discovery", was acquired only following the director's "fortuitous" visit to an exhibition by the conceptual artist who died last year.

The entire collection, including works by Julian Schnabel and Jeff Koons, was built up in a similarly arbitrary and haphazard manner, "on the basis of visits by Krens to artists and exhibitions," the conservative Basque regional MP Arturo Aldecoa said this week; he did not detail individual works. Pieces were acquired on the basis of skimpy recommendations that always coincided with the opinions of the gallery or artist in question, Mr Aldecoa said, according to Bilbao's daily El Correo yesterday.

One star exhibit, a work by Anselm Keifer created to complement the huge scale of the galleries, came to Bilbao not as the acquisition it desired, but only on long-term loan: "a loss, a sacrifice", Mr Durana reported.

The revelations emerged during investigations into a scandal that broke in April over the embezzlement of €500,000 (£400,000) by the museum's former financial director, Roberto Cearsolo, and into €6m losses caused by disastrously misjudged currency deals.

Mr Aldecoa agreed the collection's value had appreciated, but insisted the works were overpriced when bought.

Thomas Krens: Courting controversy

Thomas Krens, 61, burst on the European scene with his unlikely championing of rundown Bilbao to host a new Guggenheim museum. He fought unsuccessfully to bring Picasso's Guernica to Bilbao and courted crowds and controversy with exhibitions celebrating Armani, and the Art of the Motorcycle. After 20 years running the Solomon R Guggenheim foundation, he quit last February amid allegations he was spending too much money on satellites, neglecting the New York flagship.